1. Field of the Invention
This invention relates to the polymerization of olefins. More particularly, this invention relates to a process having catalyst compositions which are useful for polymerizing one or more monomers comprising ethylene to polymers having an intermediate molecular weight distribution and a good balance of physical properties.
2. Description of the Prior Art
It is known that catalysts of the type variously described as coordination, Ziegler, Ziegler-type, or Ziegler-Natta catalysts are useful for the polymerization of olefins under moderate conditions of temperature and pressure. It is also known that the properties of the polymers obtainable by the use of such catalysts, as well as the relative economies of the processes used to prepare the polymers, vary with several factors, including the choice of the particular monomers, catalyst components, polymerization adjuvants, and other polymerization conditions employed.
During the years since Ziegler catalysts were first publicly disclosed, there has been a considerable amount of research conducted on the use of such catalysts; and numerous publications have resulted from that research. These publications have added much to the knowledge of how to make various types of olefin polymers by various types of processes. However, as is apparent from the amount of research on Ziegler catalysis that is still being conducted throughout the world, as well as the number of patents that are still being issued to inventors working in the field of Ziegler catalysis, the means of attaining certain results when polymerizing olefins with Ziegler catalysts are still frequently unpredictable. The fact that this situation exists is sometimes due to the need to obtain a previously-unattainable combination of results; occasionally due to difficulties in obtaining the same results in a commercial-scale apparatus as in a laboratory-scale reactor; and often due to a polymerization parameter's having an effect, or side-effect, in a given type of polymerization process that is different from effects achieved by its use in prior art processes of a different type.
One aspect of Ziegler catalysis in which the need for further research has been found to exist has been in the field of preparing ethylene polymers having an intermediate molecular weight distribution and a good balance of physical properties. Such polymers have particular application in the production of sheets and certain articles that are formed by blow molding, e.g., containers for milk, and--like other polymers intended for commercial use--are desirably prepared by a process which is as economical as possible as well as being capable of producing a polymer having the desired properties.
There are, of course, known processes for preparing resins having an intermediate molecular weight distribution by polymerizing ethylene with the aid of Ziegler catalysts. However, the known processes typically suffer one or more of the disadvantages of lack of economy, inability to produce polymers having a suitable balance of properties, and/or unreliability in producing such polymers--particularly in commercial-scale operations. The processes described in Monsanto's British Pat. No. 1,489,410 are among such processes. Actually, from the teachings of the patent and what can be learned from repeating its examples, it is apparent that Monsanto's processes were designed for the preparation of blow-molding resins having broader molecular weight distributions than are desired for many blow-molding resins. However, with the use of technology not taught by Monsanto, the processes can sometimes be manipulated to produce resins having intermediate molecular weight distribution as well, and they appear to be better than many known processes for preparing blow-molding ethylene polymers in some respects, e.g. in their use of supported Ziegler catalysts having a vanadium component and in their ability to produce polymers having desirable properties in laboratory-scale operations. It has been found, however, that they are unreliable in producing polymers having intermediate molecular weight distributions.
Attempts to make Monsanto's processes more reliable in producing polymers having suitable and predictable intermediate molecular weight distributions have heretofore been unsuccessful. Polymers having a variety of molecular weight distributions ranging from broad to narrow have been produced by modifying Monsanto's teachings in various ways, but a "tunable" variable, i.e., a component that can be varied in a substantially linear way so as to permit the production of polymers having a predictable variety of intermediate molecular weight distribution has not previously been discovered.
U.S. Pat. No. 4,003,712 by Miller teaches a gas-phase fluidized bed system and process which are capable of being scaled up to commercial size and, being solvent-free, would be less expensive than processes which use solvents or liquid diluents. However, Miller's silyl chromate catalyst does not give polymers of the desired molecular weight distribution and good balance of physical properties. His system contains some features which tend to shorten commercial "on-stream" time. He does not teach how to avoid polymer buildup on reactor surfaces, a phenomenon variously referred to as "coating", "fouling", or "sheeting".
U.S. Pat. No. 3,202,645 to Yancey presents a process for polymerizing and copolymerizing alpha mono and di-olefins by a catalyst composition comprising (a) the product of the reaction between a compound of a metal chosen from the group consisting of the metals of Group IIb and IIIb (where the group numbers correspond to the Mendeleev Periodic Table) and hydroxyl groups on the surface of a finely-divided particulate inorganic solid, preferably finely-divided silica or alumina, and (b) a halide-type compound of a Group IVa, V, VIa, VIIa, or period 4 of Group VIII metal. The polymerization or copolymerization reaction can be effected at suitable temperatures within the range of from about -25.degree. C. to about 250.degree. C., and pressures ranging from below atmospheric upwardly to any desired maximum pressure, for example, 30,000 p.s.i.g. or even higher pressures. U.S. Pat. No. 3,718,636 to Stevens et al teaches obtaining polyolefins having a wide distribution of molecular weights through the use of a catalyst comprising an organometallic compound, and a solid complex component obtained by reacting a solid bivalent metal compound with an impregnation agent which consists of an organometallic compound, separating the solid reaction product, and reacting the solid reaction product with a halogenated derivative of a transition metal. Stevens et al teaches in U.S. Pat. No. 3,787,384 another catalyst suitable for use in olefin polymerization and olefin copolymerization which comprises
(a) at least one organometallic compound, and
(b) a solid catalytic component obtained by reacting a support composed of silica, alumina or both silica and alumina with a compound of the formula MR.sub.n X.sub.m-n in which M is aluminum or magnesium, R is a hydrocarbon radical containing 1 to 20 carbon atoms, X is hydrogen or a halogen, m is the valence of M, and n is a whole number not less than 1 nor greater than m, separating the solid product of the reaction, reacting said product with an excess of a halogen-containing transition metal compound, and separating the solid reaction product. U.S. Pat. No. 3,925,338 to Ort teaches that control of particle size of olefin polymers produced by gas-phase polymerization of at least one olefin using Ziegler-type catalysts deposited on solid supports in a fluidized-solids operation is effected by controlling the particle size of the catalyst support. U.S. Pat. No. 4,232,140 also to Ort discloses the use of trichlorofluoromethane as a promoter in the polymerization and copolymerization of ethylene with supported Ziegler-type vanadium compound/alkylaluminum compound catalysts in the presence of hydrogen. Ort finds that polymer yields with his supported vanadium-based catalysts are too low for commercial viability unless the catalyst is promoted to high yield with the trichlorofluoromethane promoter. Ort does not teach or suggest how to avoid reactor fouling.
U.S. Pat. No. 4,397,762 to Johnstone teaches a supported Ziegler catalyst prepared by the following steps, carried out under anhydrous conditions:
(a) reacting a hydroxyl groups-containing support material comprising magnesium silicate or silica and magnesia with one or more organo-metallic compounds having the general formula MR.sup.1.sub.a Q.sub.b-a wherein M is a metal which is aluminum, boron, lithium, zinc or magnesium, R.sup.1 is a hydrocarbyl group, Q is halogen or an oxyhydrocarbyl group, b is the valency of M and a is an integer from 1 to b, PA0 (b) removing unreacted organometallic compound, if any, from the produced solid material, PA0 (c) impregnating the solid material obtained from step (b) with one or more halogen-containing transition metal compounds wherein the transition metal(s) comprise titanium, vanadium or zirconium. PA0 (a) drying an inorganic oxide having surface hydroxyl groups to form a support that is substantially free of adsorbed water; PA0 (b) reacting the surface hydroxyl groups of the support with at least about 0.5 mol, per mol of surface hydroxyl groups, of at least one organometallic compound corresponding to the formula R.sub.x MR'.sub.y R".sub.z, wherein M is a metal of Group III of the periodic table, R is a alkyl group containing 1 to 12 carbon atoms, R' and R" are independently selected from the group consisting of H, Cl, and alkyl and alkoxy groups containing 1 to 12 carbon atoms, x has a value of 1 to 3, and y and z both represent values of 0 to 2, the sum of which is not greater than 3-x; PA0 (c) reacting the thus-treated support with at least about 0.001 molar proportion, per mol of organometallic compound, of a vanadium component consisting essentially of a mixture of about 10-90 mol percent of VOCl.sub.3 and, correspondingly, about 90-10 mol percent of VCl.sub.4 ; PA0 (d) feeding the product of step (c) into a gas-phase reaction zone; PA0 (e) feeding, separately and independently of the feeding step (d), a trialkylaluminum into the gas-phase reaction zone in order to form a bed in the gas-phase reaction zone which comprises the product of step (d) and the trialkylaluminum; PA0 (f) fluidizing the bed of step (e) at a pressure of between about 0.7 and 4.2 MPa and a temperature of between about 50.degree. to 120.degree. C. by diffusing underneath the bed of step (e) a gas mixture comprising ethylene, hydrogen, and chloroform at a rate sufficient enough to give a linear gas velocity in the bed of step (e) of between about 15 to 60 cm/sec; PA0 (g) removing particulate polymerized substantially ethylene particles from the reaction zone; and PA0 (h) recycling unreacted gas mixture of step (f) from the top of the reaction zone to the bottom of the reaction zone.
What is still needed is a process employing a catalyst which (a) is suitable for use in a gas-phase polymerization process, (b) has sufficient activity to be economically attractive, (c) contains a tunable variable that permits the production of ethylene polymers having predictable intermediate molecular weight distributions and a good balance of physical properties, (d) does not cause reactor wall fouling, and (e) a gas-phase fluidized bed process which allows the catalyst to perform at its full potential at commercial scale.